The following methods have been typically known as ways of coating a metallic substrate with hydroxyapatite.
(1) A plasma spraying method using hydroxyapatite as a starting material (see, for example, KOKAI (Japanese Unexamined Patent Publication) No. 58-39533, KOKAI No. 62-34566). PA1 (2) A method comprising the steps of preparing a powder formed mainly of a calcium salt and a phosphorus salt as a starting material, applying a coating film of the powder onto a metallic substrate by the plasma spraying technique, and immersing the coated metallic substrate in an atmosphere of steam or in water thereby converting the powder film into hydroxyapatite (see, for example, KOKAI No. 63-93851). PA1 (3) A method comprising the steps of applying to the surface of a metallic substrate an organic-solvent coating liquid having an organic calcium compound and an organic phosphorus compound dissolved therein and heating and firing the coated metallic substrate thereby forming a hydroxyapatite coating film on the metallic substrate (thermal decomposition method) (see, for example, KOKAI No. 64-86975).
The method (1) mentioned above which uses hydroxyapatite as the starting material for plasma spraying proceeds through the steps of heating and cooling because of the operating principle of plasma flame spraying and tends to submit the hydroxyapatite to decomposition and, therefore, entails the following problems.
(a) Alkali components such as calcium oxide and tetracalcium phosphate persist as residues in the coating film. PA2 (b) An amorphous phase occurs in the coating film. PA2 (c) Since the alkali components and the amorphous layer mentioned above have high degrees of solubility and the coating layer of hydroxyapatite (HAP) itself succumbs to solution readily, the produced composite poses the problem of betraying deficiency in lasting biostability, yields to embrittlement in the course of solution, possibly suffers from a lopical increase in pH in a living body, and entails a problem of biocompatibility. PA2 (d) Regarding the HAP layer, since the IR absorption spectrum of the-produced composite after separation therefrom of the HAP layer in a crushed form shows no discernible absorption peak originating in the hydroxyl group, this HAP layer does not deserve to be called a stoichiometric hydroxyapatite and poses the problem of offering no sufficient chemical stability.
The method (2) mentioned above has the problem of not only rendering it difficult to form a single apatite phase from the calcium salt and phosphorus salt by the solid phase reaction recording to the plasma spraying technique but also submitting the reaction to the onset of decomposition. Even the immersion in an atmosphere of steam or in water lends no impetus to the reaction for conversion into a single hydroxyapatite phase but entails the disadvantage that the formation of calcium carbonate and other substances ensues, the solution advances, and the coating layer embrittles.
The thermal decomposition method (3) mentioned above, similarly to the plasma spraying method, has the problem of tending to form an amorphous phase of calcium oxide and tetracalcium phosphate and rendering it difficult for the coating layer to be converted into a single phase of stoichiometric hydroxyapatite. And, owing to the fact that the substrate itself is fired in the open air, this method also has the problem of giving rise to an oxide coating layer between the substrate and the HAP coating layer and, depending upon the firing conditions, the oxide coating layer becomes brittle and when the HAP coating layer is completely dissolved in a living body and consequently exposed to the vital tissue, inducing the occurrence of macrophage and foreign giant cells and causing a foreign reaction.
Besides the methods cited above, methods such as sputtering and chemical vapor deposition are available for the coating of a metallic substrate with hydroxyapatite. These methods, however, have not been technically established as evinced by the fact that they experience great difficulty in obtaining calcium phosphate of high crystallinity.